Earth-based vehicle

ABSTRACT

A vehicle is provided including a main frame structure, a track assembly mounted to the main frame structure and having an single, endless ground-engaging track extending in a longitudinal direction. A pair of steerable ground-engaging wheels flank the track, constructed and arranged to exert a ground bearing pressure sufficient to change a direction of travel of the track in response to turning of the wheels to steer the vehicle. Steering structure is operatively associated with the wheels to turn the wheels. Power drive structure is mounted with respect to the main frame structure and is constructed and arranged to drive the wheels and the track simultaneously so as to move the vehicle along the ground. The wheels are constructed and arranged to be moved with respect to the main frame structure so as to change a position thereof with respect to the main frame structure, thereby permitting driving, steering and ground pressure of the vehicle to be adjusted in accordance with ground conditions.

The present application is a continuation of U.S. application Ser. No.09/200,971, filed Nov. 30, 1998, now U.S. Reissue Pat. No. 37,098. Bothapplications are reissue applications of original U.S. Pat. No.5,615,748, application Ser. No. 08/598,530, filed Feb. 8, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to vehicles and, more particularly, to anearth-based carrier vehicle which is constructed and arranged to put alltractive forces to the ground, thereby permitting the vehicle tonegotiate terrain having a variety of soil conditions.

Conventional vehicles such as loaders and/or bulldozers are usuallydriven by tracks or by wheels. Typical track vehicles employ a pair ofspaced steel or rubber endless tracks which are driven to move thevehicle over the ground. Wheeled vehicles typically include a pair offront wheels and a pair of rear wheels which are driven to propel thevehicle. The wheels are generally large and have tread designs which aidin moving the vehicle over sand, clay and mud. These conventionalvehicles, although capable of moving over terrain having a variety ofsoil conditions, may often get stuck, since all of the tractive forcesand propelling surfaces are not always put to the ground.

Recently, environmental concerns have been raised due to the disruptionof the earth's crust by use of the conventional loader/dozer typevehicles. For example, in the tree harvesting industry, constructionindustry and/or the agricultural industry, the operation of vehicles ofthe type described may cause significant damage to the earth's crust.Damage can occur by the formation of ruts or by causing soil erosion.

Accordingly, there exists a need to provide an improved earth-basedcarrier vehicle which may traverse terrain having a variety of soilconditions with minimal disturbance of the earth's crust.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an earth-based carriervehicle to fulfill the need referred to above. In accordance with theprinciples of the present invention, this objective is obtained byproviding a vehicle including a main frame structure, a track assemblymounted to the main frame structure and having an single, endlessground-engaging track extending in a longitudinal direction. A pair ofsteerable ground-engaging wheels are mounted with respect to the framestructure so that each wheel flanks the track. The wheels areconstructed and arranged to exert a ground bearing pressure sufficientto change a direction of travel of the track in response to turning thewheels so as to steer the vehicle. Steering structure is operativelyassociated with the wheels for turning the wheels. Power drive structureis mounted with respect to the main frame structure and is constructedand arranged to drive the wheels and the track simultaneously so as tomove the vehicle along the ground.

The wheels are constructed and arranged to be moved with respect to themain frame structure so as to change a position thereof with respect tothe main frame structure, thereby permitting driving, steering andground pressure of the vehicle to be adjusted in accordance with groundconditions.

Other objects, features and characteristics of the present invention, aswell as the methods of operation and functions of the related elementsof the structure, and the combination of the parts and economics ofmanufacture, will become more apparent upon consideration of thefollowing detailed description and appended claims, with reference tothe accompanying drawings, all of which form a part of thisspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a earth-based carrier vehicleembodying the principles of the present invention, shown with a pair ofwheels disposed in an aft position;

FIG. 2 is schematic view of the carrier vehicle of FIG. 1, shown withthe pair of wheels moved to a fore position;

FIG. 3 is a schematic plan view of the vehicle of FIG. 1;

FIG. 4A is a schematic illustration of a portion of the hydrauliccircuit of the vehicle of the invention for driving the wheels and thetrack;

FIG. 4B is a schematic illustration of a portion of the hydrauliccircuit of the vehicle of the invention for operating steering of thevehicle along with vehicle accessories;

FIG. 5 is a schematic illustration showing the axle assembly and powerunit of the vehicle mounted on a movable member;

FIG. 6 is a schematic illustration showing another embodiment of theaxle assembly and power unit of the vehicle mounted on a movable member;

FIG. 7 is partial perspective view of an end of the movable member ofFIG. 6;

FIG. 8 is a schematic illustration of the vehicle of the inventionhaving a vertically movable axle assembly;

FIG. 9 is a perspective view of an air bag device mounted to the axleassembly of the vehicle of FIG. 8;

FIGS. 10A, 10B and 10C show examples of track profiles for the track ofthe vehicle of the invention; and

FIG. 11 is a schematic side view of a tandem carrier vehicle embodyingthe principles the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

Referring to the drawings, an earth-based carrier vehicle is showngenerally indicated at 10, which embodies the principles of the presentinvention.

The vehicle 10 includes a main frame structure, generally indicated at12, in the form of a vehicle chassis, having fore and aft ends, 14 and16, respectively. In the illustrated embodiment, an operator compartment18 is mounted to the fore end 14 of the frame structure 12, while awinch or other attachment assembly, generally indicated at 20, ismounted to the aft end 16.

In accordance with the principles of the present invention, the vehicle10 includes a track assembly, generally indicated at 22, mounted to theframe structure 12. The track assembly 22 may comprise one or moreendless track belt units. In the illustrated embodiment, the trackassembly 22 is in the form of a single, centrally mounted track unithaving a conventional ground-engaging endless track or belt 26 extendingin the longitudinal direction. The track assembly 22 may be ofconventional configuration; for example, Model MP73, manufactured byCaterpillar Industrial Products Inc., of Peoria, Ill., may be employed.Two wheel rear drive or an all wheel drive track assembly is within thecontemplation of the invention. It is preferred that the track assemblybe powered, such as the conventional lug and sprocket-type arrangement,whereby belt or track lugs engage track wheel sprockets. Alternatively,the track assembly may be of the friction-type whereby the track or beltfrictionally engages the track wheels of the assembly.

In the illustrated embodiment, two hydraulic motors MA and MB (shownschematically in FIG. 4A) are provided, with one motor coupled to anassociated drive wheel 23 of the track assembly 22 for driving the track26. The motors MA and MB are conventional, such as a Heavy DutyXL-1120cc radial piston motor, with hub mount, manufactured by RotaryPower. The motors MA and MB are preferably of the fixeddisplacement-type but are capable of “free wheeling”, unpoweredoperation.

It is within the contemplation of the invention to employ a conventionalmechanical transmission to drive the track 26 in lieu of hydraulicpower.

The track 26 may be a rubber or steel track, however, rubber ispreferred. The bounds or inner periphery of the track 26 defines aninterior space or envelope 28, the function of which will becomeapparent below. The track 26 may have various profiles, as shown by thetracks 26, 126 and 226, respectively, in FIGS. 10A-10C.

An axle assembly, generally indicated at 30, is mounted with respect tothe frame structure 12. The axle assembly 30 has opposing ends 32 and34, respectively, which are disposed generally transverse to thelongitudinal extent of the track 26 outside of the envelope 28 atpositions adjacent the track 26. As shown in FIG. 3, a portion of theaxle assembly 30 extends through the envelope 28 of the track assembly22. A pair of steerable wheels are provided, with one wheel 36 beingcoupled to a respective end of the axle assembly 22. Thus, as shown, thewheels flank the track 26 and are arranged with respect to the track sothat the wheels engage the ground surface at a position deeper into theground than the engagement position of the track with the groundsurface, the function of which will become apparent below.

The axle assembly 30 is a conventional, of the driving and steeringtype, and may be, for example, a John Deere TeamMate™II 1150 SeriesSteerable Axle manufactured by Deere Power Systems of Waterloo, Iowa.For the steering function, the TeamMate™II 1150 Series Steerable Axleincludes steering structure in the form of dual double-acting hydrauliccylinders and a tie-rod. It can be appreciated that the wheel mountingassembly or axle assembly can be of any construction which provides bothdriving and steering of the wheels 36.

In the illustrated embodiment, one hydraulic motor MC (FIG. 4A) iscoupled to the axle assembly 30 differential to drive the wheels 36. Themotor MC is conventional, such as a Series 90 75cc 2-speed motormanufactured by Sauer Sundstrand Company of Ames, Iowa. The motor MC ispreferably a variable displacement-type motor and is operated at eithermaximum or minimum displacement as determined by the operator, as willbe explained in more detail below. Although one motor is used in theillustrated embodiment, it is within the contemplation of the inventionto provide the appropriate number of hydraulic motor(s) to operate theparticular axle assembly selected.

Although a hydraulic motor is used to drive the wheels 36, it is withinthe contemplation of the invention to employ mechanical means such as atransmission coupled to the drive shaft of the axle assembly 30 to drivethe wheels 36 in the known manner. Further, although only a pair ofwheels are provided in the illustrated embodiment, it can be appreciatedthat front and rear wheel pairs may be provided of such arrangement toprovide both driving and steering functions.

In the illustrated embodiment, steering of the wheels 36 is achievedhydraulically. Thus, the steering structure includes a hydraulicsteering unit, generally indicated at 40 (FIGS. 1 and 4B). The steeringunit 40 is conventional and may be, for example, the Danfoss OSPB ONopen center, non-reaction steering unit. Thus, in the illustratedembodiment, there is no mechanical connection between the steeringcolumn 42 and the steered wheels 36. Instead, in the conventionalmanner, hydraulic hoses are connected between the steering unit 40 andthe hydraulic steering cylinders 39 of the axle assembly 30. Onehydraulic cylinder 39 is associated with each wheel 36. With referenceto FIG. 4B, when the steering wheel 44 is turned, the steering unit 40meters an oil volume proportional to the rate of rotation of thesteering wheel 44. This volume is directed to the appropriate side of asteering cylinder 39 of the steering structure, while displaced oil isdirected to the reservoir 46. Although hydraulic power is employed toturn the wheels 36, it is within the contemplation of the invention toemploy mechanical means such as linkages, as steering structure, to turnthe wheels 36. Further, it is within the contemplation of the inventionto employ a center-pivoted rigid axle for steering the wheels 36.

The wheels 36 preferably include rubber tires. However, it can beappreciated that steel wheels can also be employed. Thus, it is withinthe contemplation of the invention to employ a rubber track and rubbertires, or a steel track and steel wheels, or rubber tires and a steeltrack, or steel wheels and a rubber track. The material used for thetrack and wheels/tires may be selected based upon the particular vehicleapplication.

Power drive structure, generally indicated at 50, includes the poweroperated equipment necessary to drive the wheels 36 and to drive thetrack 26. In the illustrated embodiment, the power drive structureincludes a power unit, generally indicated at 51, including an internalcombustion engine 52, a pump drive gear box 54 coupled to the engine 52and a hydrostatic transmission pump 56 coupled to the gear box 54. Thepower unit 51 is mounted within the envelope 28. Motors MA, MB and MCare also considered part of the drive structure. The engine 52 drivesthe hydraulic transmission pump 56, which is fluidly connected to thetwo track hydraulic motors MA and MB to drive the track 26, and fluidlyconnected to the hydraulic motor MC of the axle assembly 30 to drive thewheels 36. The hydraulic pump 56 is conventional and may be, forexample, a Series 90, 75 cc/rev hydrostatic pump, manufactured by SauerSundstrand Company. The engine 52 may be conventional, such as the Model4045T PowerTech 4.5L engine manufactured by Deere Power Systems ofWaterloo, Iowa. In the illustrated embodiment, the pump gear box 54,mounted to engine 52 is a Series 28000 Double 6 inch type, manufacturedby Funk Manufacturing Company. The gear box 54 is provided so as tooperate two pumps, pump 56 and pump unit 57 off the engine 52. Pump unit57 (FIG. 4A) is a tandem gear pump employed to operate the steering unit40 (FIG. 4B) of the vehicle and any vehicle accessories such as a dozerblade 110 (FIG. 8) mounted on the vehicle, as explained in more detailbelow. Pump unit 57 is conventional, such as a Barnes gear pump ModelG-20, for operating the accessories, with a Barnes Series 900 piggybackgear pump coupled thereto, for operating the steering unit 40.

As noted above, it can be appreciated that in lieu of or in combinationwith the hydrostatic pump 56 and hydraulic motors to drive the track 26and wheels, the power drive structure may include mechanical means suchas a mechanical transmission to facilitate driving the track 26 and thewheels 36.

The vehicle 10 is driven by the hydrostatic transmission pump 56delivering hydraulic fluid such as oil to the two track motors MA and MBand the axle motor MC. Drive speed is determined by the number of motorsselected to operate at the same time. Selection of operation of themotors is performed through an electrical switch operated, solenoidcontrolled, high pressure poppet valves, (S1-S9 in FIG. 4A) which areconventional and operated by switches 47 mounted within the operatorcompartment 18.

Driving the vehicle 10 can be appreciated with reference to thehydraulic circuit shown in FIG. 4A and with reference to TABLE 1.

TABLE 1 S1 S2 S3 S4 S5 S6 S7 S8 S9 LOW SPEED X X X X X X 2ND SPEED X X X3RD SPEED X X 4TH SPEED X X X

TABLE 1 shows the particular solenoid valves, S1-S9, which are energizedin various combinations resulting in various vehicle speeds. At lowspeed, solenoid valves S1, S2, S4, S5, S7 and S8 are energized. Thus,the axle motor MC is at maximum displacement and the track motors MA andMB are both operating. At this speed, solenoid valves S1 and S2 areenergized only to limit wheel spin. At the second speed, solenoid valvesS4, S5, and S9 are energized. Thus, the axle motor MC is at maximumdisplacement and the track motor MA is operating while track motor MB isfree wheeling. At the third speed, solenoid valves 6 and 9 areenergized. The axle motor MC is at maximum displacement and the trackmotors MA and MB are free wheeling. At the fourth speed and fastestspeed, solenoid valves S3, S6 and S9 are energized. Thus, the axle motorMC is at minimum displacement and the track motors MA and MB are freewheeling. Thus, it can be appreciated that the wheels 36 and track 26are driven simultaneously to propel the vehicle.

Forward, stop and reverse control is determined by an operator controllever position. The lever 60 is connected preferably via a push-up cable(not shown) to a manual direction servo control valve located on thehydrostatic transmission pump 56 to control the pump 56. However, it canbe appreciated that the pump 56 may be controlled electrically. Movementof the lever 60 in the forward direction from a middle or stop positionwill produce forward vehicle movement. The further forward the lever 60is pushed, the faster the travel rate of the vehicle becomes. Movementof the lever 60 in the reverse direction or towards the operatorproduces rearward travel in a similar way. Since the vehicle ishydraulically operated, oil flow and pressure are factors in controllingspeed, torque, and thus tractive effects on the wheels 36 and track 26.

It is within the contemplation of the invention to provide an additionalvalve (not shown) into the hydraulic circuit to provide a parking brake.The brake can be mounted on the track drive and can be powered off andspring applied.

The remainder of the hydraulic circuit as shown in FIG. 4B is variableand applies to the support circuitry for use with one or moreattachments fixed to the vehicle. For example, FIG. 8 shows a vehicle100 of the invention including a dozer blade 110. As shown in FIG. 4B,the pump unit 57 operates the motor 21 of the winch assembly 20 and thehydraulic cylinders 111, 113 and 115 of the dozer blade 110 via acontrol valve module, generally indicated at 58. The Danfoss PVG 32valve module may be used as the control valve module 58. With reference! to the drawings, it can be appreciated that the tandem pump unit 57 ofFIG. 4A is coupled to the hydraulic circuit portion of FIG. 4B at pointsC and D, while the steering portion of the circuit of FIG. 4B is coupledto the circuit of FIG. 4A at E.

The axle assembly 30 may be fixed with respect to the main framestructure 12. When the axle assembly 30 is fixed, the vehicle is steeredby the wheels 36 via the steering unit 40, in the manner discussedabove. In the illustrated embodiment (FIG. 1), with respect to the mainframe structure, the wheels 36 are disposed at a position verticallylower than a position of the track 26 so that the wheels 36 will engagethe ground at positions slightly deeper into the earth's crust than theposition of the track's 26 ground contact. Further, for the bestresults, the wheels 36 should be disposed at the center of gravity ofthe vehicle to increase the tractive force at the wheels 36. Thus, eventhough the driven track 26.will try to maintain movement of the vehiclein the particular direction of vehicle travel, when the wheels 36 areturned to steer the vehicle, the track 26 will follow. This occurs sincethe wheels 36 are constructed and arranged to exert ground bearingpressure sufficient to move or nudge the track 26 and thus, change adirection of travel of the track in response to turning the wheels tosteer the vehicle. Thus, upon turning the wheels to steer the vehicle,the driven track 26 will respond by sliding over the ground in thedirection that the vehicle is turning.

It is preferable to mount the axle assembly 30 so as to be movable withrespect to the main frame structure 12 in at least the verticaldirection. Thus, the axle assembly 30 may be fixed fore and aft, yet bemounted for generally vertical movement so as to adjust the position ofthe wheels 36 with respect to the frame structure 12, the function ofwhich will become apparent below.

As noted above, FIG. 8 shows another embodiment of the carrier vehicleof the invention. The vehicle 100 is employed as a bulldozer having adozer attachment 110. The axle assembly 130 of the vehicle 100 ismovable vertically. With this arrangement, distribution of weight andtractive effort between the wheels 36 and the track 126 for improveddriving and steering of the vehicle 100 can be achieved. As shown inFIG. 9, to move the axle assembly 130 vertically, a pneumatic air bagdevice, generally indicated at 132, may be top mounted to an upper framemember of the frame structure 112 and bottom fixed to a hinged saddle134 that extends to the back of the axle assembly 130. The saddle 134 iscoupled to the frame structure 112 via hinge pins 136. Upon inflation,the air bag of the device 132 pushes the axle assembly 130 and thus thewheels 36 downwardly within the envelope, raising the track 126 off theground at the fore end of the vehicle 100, increasing the ground bearingpressure at the wheels. Thus, the wheels 36 and only the aft end of thetrack 126 are in ground contact. In this position, it can be appreciatedthat turning of the wheels 36 would result in turning of the vehicle 100about a small turning radius since the tractive forces are only at thewheels 36 and at the aft end of the track 126. Since a portion of thedriven track 126 is out of ground contact, improved mobility of thevehicle 100 is achieved. Limit stops 138 (FIG. 9) may be providedbetween the air bag device 132 and the axle assembly 130 for restrictingoscillatory movement of the axle assembly 130.

When the air bag of the device 132 is caused to deflate, the axleassembly 130 will move to a vertical position such that the track 126and the wheels 36 are each in contact with the ground.

It can be appreciated that other devices, such as hydraulic or aircylinders (not shown) coupled to, for example, the center pivoting mountof the axle assembly 130, may be used to raise or lower the axleassembly 130 within the envelope 28 of the track assembly 122.

In the preferred embodiment as shown in FIG. 2, the axle assembly 30 andthe power unit 51 are fixedly mounted on a movable member, generallyindicated at 53, so as to be movable within the envelope 28. FIG. 5 is aschematic view of the axle assembly 30 and power unit 51 mounted on themovable member, generally indicated at 53 so as to advantageously changethe fore and aft center of gravity per load, attachment, or tocompensate for various surface conditions which may prevail, and tochange the driving and steering geometry in any working conditions.

In the illustrated embodiment, the movable member 53 is in the form of aflat plate mounted for movement along a tract frame, generally indicatedat 62. The tract frame 62 is fixed to the frame structure 12 within theenvelope 28. Ball bearing V-shaped rollers or cam followers 64 areemployed which permit guided movement of the movable member 53 along thetract frame 62. At least one hydraulic cylinder 66 is fixed at one endto the frame structure 12. The piston end 67 of the cylinder 66 iscoupled to an end of the movable member 53 such that movement of thepiston moves the movable member 53 fore and aft along the tract frame62. The cylinder(s) 66 may be incorporated into the hydraulic circuit ofFIG. 4B and operated by the pump unit 57. The tract frame 62 is disposedalong an axis B, which is disposed at an angle with respect tohorizontal (axis A). Thus, as shown in FIG. 1, when the movable member53 is in the aft position, the axle assembly 30 and the power unit 51are disposed at a position vertically higher than when the movablemember 53 is in its fore position (FIG. 2). This permits positioning theload bearing weight and the tractive forces of the wheels 36 so as tocorrespond with the area and tractive forces of the track 26 therebyallowing driving, steering and ground pressure to be adjusted. In thefore position, the wheels are disposed so as to raise the front portionof the track 26 off the ground, applying more ground bearing pressure onthe wheels 36, so as to dramatically change the driving and steeringgeometry of the vehicle 10. Thus, the vehicle 10 can be stably propelledand steered with minimal disturbance to the earth's crust.

It can be appreciated that the movable member 53 may be mounted formovement in many different ways. For example, as shown in FIGS. 6 and 7instead of using bearings to permit movement of the movable member, themovable member 153 may be a flat plate mounted within a guide track 158.The track 158 may be fixed to or formed as an integral part of the framestructure 12. The guide track 158 is disposed along the inclined axis B(FIG. 5). The movable member 153 may be adapted to slide within thetrack fore and aft. The inside of the track may be lined with TEFLON orhigh molecular plastic 160 to reduce friction between the track and themovable member 153. Hydraulic cylinders 66 or other moving means may becoupled to the movable member to move the movable member 153 fore andaft within the guide track in the manner discussed above.

Since the vehicle of the invention may traverse uneven terrain, theremay be a need to compensate for any vehicle yaw. If one wheel rises withrespect to the other, for example, if one wheel runs over a stump,oscillation of the vehicle and track mount may occur. This oscillationor yaw can be compensated for by providing, for example, shockabsorbers, springs, or rubber blocks between the frame structure andwheel axles.

With reference to FIG. 11, a tandem carrier vehicle 200 is shown,embodying the principles of the present invention. The tandem vehicle200 includes a front powered vehicle 210, which is substantially similarto the vehicle 10 of FIG. 1. However, vehicle 210 is provided with anarticulated connection 214 at the aft end thereof. A rear vehicle 212 iscoupled to vehicle 210 at the connection 214. It is within thecontemplation of the invention to provide vehicles 210 and 212 withtrack profiles which are shorter than the profile of the single vehicle10 of FIG. 1. Further, vehicle 210 may be provided with a fixed axleassembly 230, a vertically movable axle assembly or a movable axleassembly and movable power unit, with driven and steerable wheels, asdiscussed above. Vehicle 22.2 preferably has a fixed, axle assembly 232.The track and wheels of the rear vehicle 212 are driven by the powerdrive structure 251 of vehicle 210. Thus, the rear vehicle 212 includesthe appropriate hoses originating from vehicle 210 providing thehydraulic power necessary to drive the track and wheels of the first andsecond vehicles, simultaneously.

It can be seen that the vehicle of the present invention may be drivenand easily maneuverable over a variety of terrains without significantlydamaging the earth's crust. Further, the vehicle can be configured inmany different ways by providing the appropriate attachments. Forexample, winches, dozer and loader attachments can be provided in anydesired manner so as to optimize the performance based upon theparticular vehicle application.

It can be appreciated that the vehicle of the invention can be modifiedwithout departing from the principles of the invention. For example,although hydraulic power is disclosed for operating the vehicle, otherknown modes of power can be used. It is also within the contemplation ofthe invention to adjust the size of the vehicle so as to be employed asa child's toy. Thus, manual power, such as pedaling, or battery powermay be used to propel such a vehicle. Further, the vehicle may beadapted for use as a recreational vehicle.

In addition, although the engine and/or power unit is disclosed as beingmounted within the envelope of the track, it is within the contemplationof the invention to mount the engine and/or power unit anywhere on thevehicle. For example, the engine and/or power unit may be mounted nearthe operator's compartment. Further, the engine and/or power unit neednot be movable, but may be fixed with respect to the main framestructure.

It thus will be seen that the objects of this invention have been fullyand effectively accomplished. It will be realized, however, that theforegoing preferred embodiments of the present invention have been shownand described for the purposes of illustrating the structural andfunctional principles of the present invention and are subject to changewithout departure from such principles. Therefore, this inventionincludes all modifications encompassed within the spirit of thefollowing claims.

1. A vehicle comprising: a main frame structure, a track assemblymounted to said main frame structure and having an endlessground-engaging track extending in a longitudinal direction, a pair ofsteerable ground-engaging wheels mounted with respect to the main framestructure such that each said wheel of said pair flanks said track, saidwheels being constructed and arranged to exert ground bearing pressuresufficient to change a direction of travel of said track in response toturning of said wheels so as to steer the vehicle, steering structureoperatively associated with said wheels to turn said wheels, and powerdrive structure mounted with respect to said main frame structure andconstructed and arranged to drive said wheels and said tracksimultaneously so as to move the vehicle along the ground.
 2. Thevehicle according to claim 1, wherein said wheels are constructed andarranged to be moved with respect to said main frame structure so as tochange a position thereof with respect to the main frame structure,thereby permitting driving, steering and ground bearing pressure of thevehicle to be adjusted in accordance with ground conditions.
 3. Thevehicle according to claim 1, wherein said wheels are coupled to an axleassembly at opposing ends of said axle assembly, said axle assemblybeing mounted with respect to said main frame structure so as to bedisposed generally transverse to the longitudinal extent of said track.4. The vehicle according to claim 3, wherein bounds of said track definean interior space, said axle assembly being mounted with respect to saidmain frame structure such that a portion of said axle assembly extendsthrough said interior space.
 5. The vehicle according to claim 4,wherein said axle assembly is constructed and arranged to be movablewith respect to said main frame structure so as to change a position ofsaid wheels with respect to the main frame structure, thereby permittingdriving, steering and ground bearing pressure of the vehicle to beadjusted in accordance with ground conditions.
 6. The vehicle accordingto claim 5, wherein said axle assembly is movable generally verticallywith respect to said frame structure to adjust a vertical position ofsaid wheels.
 7. The vehicle according to claim 6, wherein said axleassembly is movable an amount such that said wheels lift at least aportion of said track from ground contact.
 8. The vehicle according toclaim 6, further comprising an air bag device operatively associatedwith said axle assembly for moving said axle assembly generallyvertically.
 9. The vehicle according to claim 1, wherein said framestructure has fore and aft ends and said track assembly is mountedcentrally with respect to said main frame structure between said foreand aft ends.
 10. The vehicle according to claim 4, wherein said powerdrive structure includes an engine, at least one hydrostatic pump drivenby said engine, at least one hydraulic motor coupled to said axleassembly and at least one hydraulic motor coupled to said trackassembly, said hydrostatic pump being fluidly connected to saidhydrostatic motors for driving said track and said wheels, said engineand said hydrostatic pump defining a power unit.
 11. The vehicleaccording to claim 10, wherein said power unit and said axle assemblyare mounted on a movable member, said movable member being mounted withrespect to said frame structure so as to be movable fore and aft withinsaid interior space so as to change the fore and aft center of gravityof the vehicle.
 12. The vehicle according to claim 11, wherein saidmovable member is movable such that when said movable member is disposedat a fore position, said wheels lift at least a fore end of said trackfrom ground contact.
 13. The vehicle according to claim 11, wherein saidmovable member is a flat plate mounted within a guide track so as to beslidable therein, said guide track being fixed to said main framestructure.
 14. The vehicle according to claim 13, further comprising atleast one hydraulic cylinder coupled to said flat plate for moving theflat plate within the guide track.
 15. The vehicle according to claim11, wherein said movable member is movable along a track member viabearings, said track member being fixed to said main frame structure.16. The vehicle according to claim 1, wherein said wheels include rubbertires and said track is a rubber track.
 17. The vehicle according toclaim 1, wherein said steering structure is constructed and arranged tobe operated hydraulically.
 18. The vehicle according to claim 1, whereinsaid steering structure includes a steering unit and a hydrauliccylinder associated with each said wheel to move the associated wheel,said steering unit being hydraulically operated and fluidly coupled tothe hydraulic cylinders for controlling turning movement of said wheels.19. The vehicle according to claim 1, in combination with a secondvehicle, said second vehicle including a second frame structure, asecond track assembly mounted to said second frame structure andincluding an endless ground-engaging track, and a pair of steerableground-engaging wheels mounted with respect to said second framestructure so as to flank the second track assembly, said power drivestructure being constructed and arranged to drive the track and wheelsof the first and second vehicles, simultaneously.
 20. The vehicleaccording to claim 10, wherein said hydraulic motors are constructed andarranged to be operated in such a manner that vehicle speed isdetermined by the number of motors selected to operate at one time. 21.The vehicle according to claim 1, wherein said wheels are disposed at aposition vertically lower than a position of said track relative to saidmain frame structure, so that said wheels make ground contact at pointsdeeper into the ground than points of track ground contact.
 22. Avehicle comprising: a main frame structure, a track assembly mountedgenerally centrally with respect to said main frame structure and havinga single, endless ground-engaging track extending in a longitudinaldirection, a pair of steerable ground-engaging wheels mounted withrespect to the main frame structure such that each said wheel of saidpair flanks said track, said wheels being constructed and arranged toexert ground bearing pressure sufficient to change a direction of travelof said track in response to turning of said wheels to steer thevehicle, steering structure operatively associated with said wheels toturn said wheels, and power drive structure mounted with respect to saidmain frame structure and constructed and arranged to drive said wheelsand said track simultaneously so as to move the vehicle along theground, said wheels being movable with respect to said main framestructure so as to change a position of said wheels with respect to theframe structure, thereby permitting driving, steering and ground bearingpressure of the vehicle to be adjusted in accordance with groundconditions.
 23. The vehicle according to claim 22, wherein said wheelsare coupled to an axle assembly at opposing ends of said axle assembly,said axle assembly being mounted with respect to said main framestructure so as to be disposed generally transverse to the longitudinalextent of said track.
 24. The vehicle according to claim 22, whereinsaid steering structure is constructed and arranged to be operatedhydraulically.
 25. The vehicle according to claim 22, wherein said mainframe structure has fore and aft ends and said track assembly extendssubstantially between said fore and aft ends.
 26. The vehicle accordingto claim 22, wherein said wheels are mounted for vertical movement withrespect to said main frame structure.
 27. The vehicle according to claim23, wherein said axle assembly is constructed and arranged to be movablegenerally vertically to adjust a vertical position of said wheels. 28.The vehicle according to claim 27, wherein said axle assembly is movablean amount such that said wheels lift at least a portion of said trackfrom ground contact.
 29. The vehicle according to claim 27, furthercomprising an air bag device operatively associated with said axleassembly for moving said axle assembly generally vertically.
 30. Thevehicle according to claim 23, wherein said power drive structureincludes an engine, at least one hydrostatic pump driven by said engine,at least one hydraulic motor coupled to said axle assembly and at leastone hydraulic motor coupled to said track assembly, said hydrostaticpump being fluidly connected to said hydrostatic motors for driving saidtrack and said wheels, said engine and said hydrostatic pump defining apower unit.
 31. The vehicle according to claim 30, wherein saidhydraulic motors are constructed and arranged to be operated in such amanner that vehicle speed is determined by the number of motors selectedto operate at one time.
 32. The vehicle according to claim 30, whereinbounds of said track define an interior space, said power unit and asubstantial portion of said axle assembly being disposed within saidinterior space.
 33. The vehicle according to claim 32, wherein saidpower unit and said axle assembly are mounted on a movable member, saidmovable member being mounted with respect to said frame structure so asto be movable fore and aft within said interior space so as to changethe fore and aft center of gravity of the vehicle.
 34. The vehicleaccording to claim 33, wherein said movable member is movable such thatwhen said movable member is disposed at a fore position, said wheelslift at least a fore end of said track from ground contact.
 35. Thevehicle according to claim 32, wherein said movable member is a flatplate mounted within a guide track so as to be slidable therein, saidguide track being fixed to said main frame structure.
 36. The vehicleaccording to claim 35, further comprising at least one hydrauliccylinder coupled to said flat plate for moving the flat plate within theguide track.
 37. The vehicle according to claim 33, wherein said movablemember is movable along a track member via bearings, said track memberbeing fixed to said main frame structure.
 38. The vehicle according toclaim 22, wherein said wheels include rubber tires and said track is arubber track.
 39. The vehicle according to claim 25, further comprisinga dozer blade mounted with respect to said frame structure at the foreend thereof.
 40. The vehicle according to claim 22, wherein saidsteering structure includes a steering unit operatively associated withsaid wheels for controlling steering of the vehicle.
 41. The vehicleaccording to claim 40, wherein said steering structure includes ahydraulic cylinder associated with each said wheel, said steering unitbeing hydraulically operated and fluidly coupled to the hydrauliccylinders for controlling steering of said wheels.
 42. A tandem vehiclecomprising: a first vehicle including; a main frame structure, a trackassembly mounted generally centrally with respect to said main framestructure and having a single, endless ground-engaging track extendingin a longitudinal direction, a pair of steerable ground-engaging wheelsmounted with respect to the main frame structure such that each saidwheel of said pair flanks said track, said wheels being constructed andarranged to exert ground bearing pressure sufficient to change adirection of travel said track in response to turning of said wheels tosteer the first vehicle, steering structure operatively associated withsaid wheels to turn said wheels, power drive structure mounted withrespect to said main frame structure and constructed and arranged todrive said wheels and said track simultaneously so as to move the firstvehicle along the ground, said wheels being movable with respect to saidmain frame structure so as to change a position of said wheels withrespect to the frame structure, thereby permitting driving steering, andground bearing pressure of the first vehicle to be adjusted inaccordance with ground conditions, a second vehicle coupled to an aftend of said first vehicle, said second vehicle including a second framestructure, a second track assembly mounted to said second framestructure and including an endless ground-engaging track, and a pair ofdriven and steerable ground-engaging wheels mounted with respect to saidsecond frame structure so as to flank the second track assembly, saidpower drive structure being constructed and arranged to drive the trackand wheels of the first and second vehicles, simultaneously.
 43. Atandem vehicle comprising: a first vehicle including: a main framestructure, a track assembly mounted to said main frame structure andhaving an endless ground-engaging track extending in a longitudinaldirection, a pair of steerable ground-engaging wheels mounted withrespect to the main frame structure such that said wheels flank saidtrack, said wheels being constructed and arranged to exert groundbearing pressure sufficient to change a direction of travel of saidtrack in response to turning of the wheels to steer the first vehicle,steering structure operatively associated with said wheels to turn saidwheels, and power drive structure mounted with respect to said mainframe structure and constructed and arranged to drive said wheels andsaid track simultaneously so as to move the first vehicle along theground, and a second vehicle coupled to an aft end of said firstvehicle, said second vehicle including: a second main frame structure, asecond track assembly mounted to said second main frame structure andhaving an endless ground-engaging track extending in a longitudinaldirection, a pair of steerable ground-engaging wheels mounted withrespect to the second main frame structure such that said wheels flanksaid track, said wheels being constructed and arranged to exert groundbearing pressure sufficient to change a direction of travel of saidsecond track in response to turning of the wheels of the second track,said track and said wheels of said second vehicle being operativelycoupled to said power drive structure of said first vehicle such thatsaid power drive structure of said first vehicle drives said wheels andtrack of said first vehicle together with said wheels and track of saidsecond vehicle.
 44. A vehicle comprising: a main frame structure; atrack assembly supporting said main frame structure and having anendless ground-engaging track extending in a longitudinal direction ofsaid vehicle; a pair of ground-engaging wheels mounted with respect tosaid main frame structure such that said wheels are disposed in flankingrelation on opposing sides of said track, said wheels and said trackassembly being mounted within said vehicle so as to enable said wheelsand said track assembly to be moved generally vertically relative to oneanother to shift a distribution of vehicle weight between said wheelsand said track assembly; a power drive assembly constructed and arrangedto move said wheels and said track in driving engagement with the groundso as to drive said vehicle along the ground; and steering structureoperatively associated with said wheels, said steering structure beingconstructed and arranged to control operation of said wheels so as toaffect a vehicle steering operation wherein said power drive structuremoves said wheels in driving engagement with the ground to steer saidvehicle; power-operated structure constructed and arranged to affect thegenerally vertical relative movement between said wheels and said trackassembly during said vehicle steering operation to shift saiddistribution of vehicle weight from said track assembly to said wheelsto provide said wheels with increased traction during said vehiclesteering operation.
 45. A vehicle according to claim 44, wherein saidpower-operated structure is connected to said wheels and is constructedand arranged to move said wheels generally downwardly relative to saidtrack assembly to shift said distribution of the vehicle weight.
 46. Avehicle according to claim 45, wherein said power-operated structure iscommunicated to a supply of pressurized fluid and is constructed andarranged to move said wheels generally downwardly relative to said trackassembly using said pressurized fluid.
 47. A vehicle according to claim46, wherein said supply of pressurized fluid is a supply of pressurizedhydraulic liquid and wherein said power-operated structure includeshydraulic cylinders operatively connected to said wheels.
 48. A vehicleaccording to claim 47, wherein said power drive assembly includes ahydraulic pump constructed and arranged to pressurize said supply ofpressurized liquid.
 49. A vehicle according to claim 46, wherein saidsupply of pressurized fluid is a supply of pressurized air and whereinsaid power-operated structure includes an air bag operatively connectedto said wheels.
 50. A vehicle according to claim 44, wherein saidpower-operated structure is constructed and arranged to affect thegenerally vertical relative movement between said wheels and said trackassembly to an extent sufficient to lift a substantial portion of saidtrack assembly out of ground contact about a longitudinal end portionthereof during vehicle steering operation to thereby distribute theweight of said vehicle over three spaced supporting points includingsaid longitudinal end portion of said track and each of said wheels. 51.A vehicle according to claim 50, wherein said longitudinal end portionof said track assembly is a rear end portion of said track assembly. 52.A vehicle according to claim 50, wherein said power-operated assembly isconnected to said wheels and is constructed and arranged to move saidwheels generally downwardly relative to said track assembly to shiftsaid distribution of the vehicle weight.
 53. A vehicle according toclaim 52, wherein said power-operated structure is communicated to asupply of pressurized fluid and is constructed and arranged to move saidwheels generally downwardly relative to said track assembly using saidpressurized fluid.
 54. A vehicle according to claim 52, wherein saidsupply of pressurized fluid is a supply of pressurized hydraulic liquidand wherein said power-operated structure includes a hydraulic cylinderoperatively connected to said wheels.
 55. A vehicle according to claim54, wherein said power drive structure includes a hydraulic pumpconstructed and arranged to pressurize said supply of pressurizedliquid.
 56. A vehicle according to claim 55, wherein said supply ofpressurized fluid is a supply of pressurized air and wherein saidpower-operated structure includes an air bag operatively connected tosaid wheels.
 57. A vehicle according to claim 44, wherein said wheelsare pivotally mounted to said frame for pivotal steering movement abouta pair of generally vertical axes and wherein said steering structure isconstructed and arranged to pivot said wheels about said generallyvertical axes so as to affect said vehicle steering operation.
 58. Avehicle according to claim 50, wherein said wheels are pivotally mountedto said frame for pivotal steering movement about a pair of generallyvertical axes and wherein said steering structure is constructed andarranged to pivot said wheels about said generally vertical axes so asto affect said vehicle steering operation.
 59. A vehicle according toclaim 44, further comprising a single axle extending transversely ofsaid frame, said wheels being respectively mounted on opposing ends ofsaid axle assembly.
 60. A vehicle according to claim 59, wherein saidpower-operated structure is connected between said axle and said frame,said power-operated structure being constructed and arranged to movesaid axle assembly generally downwardly relative to said frame so as tomove said wheels generally downwardly relative to said track assembly tothereby shift said distribution of the vehicle weight.
 61. A vehicleaccording to claim 60, wherein said axle is hingedly connected to saidframe.
 62. A vehicle according to claim 60, further comprising a movablemember mounted within track assembly for movement in an inclined manner,said axle being mounted to said movable member such that movement ofsaid movable member in said inclined manner moves said axle generallyvertically relative to said track assembly, said power-operatedstructure being connected between said frame and said movable member.63. A vehicle according to claim 60, wherein said power-operatedstructure is communicated to a supply of pressurized fluid and isconstructed and arranged to move said axle generally downwardly relativeto said track assembly using said pressurized fluid.
 64. A vehicleaccording to claim 59, wherein said wheels are pivotally mounted to saidaxle for pivotal steering movement about a pair of generally verticalaxes and wherein said steering structure is constructed and arranged topivot said wheels about said generally vertical axes so as to affectsaid vehicle steering operation.
 65. A vehicle according to claim 50,further comprising a single axle extending transversely of said frame,said wheels being respectively mounted on opposing ends of said axle.66. A vehicle according to claim 65, wherein said power-operatedstructure is connected between said axle and said frame, saidpower-operated structure being constructed and arranged to move saidaxle generally downwardly relative to said frame so as to move saidwheels generally downwardly relative to said track assembly to therebyshift said distribution of the vehicle weight.
 67. A vehicle accordingto claim 66, wherein said axle is hingedly connected to said frame. 68.A vehicle according to claim 67, wherein said power-operated structureis communicated to a supply of pressurized fluid and is constructed andarranged to move said axle assembly generally downwardly relative tosaid track assembly using said pressurized fluid.
 69. A vehicleaccording to claim 44, further comprising an axle assembly having wheelmounting portions disposed on opposing sides of said track assembly,said wheels being rotatably mounted to said wheel mounting portions andsaid axle assembly being constructed and arranged such that said wheelmounting portions are movable generally vertically relative to saidtrack assembly so as to allow the generally vertical relative movementbetween said wheels and said track assembly; said power-operatedstructure being operatively connected between said frame and said axleassembly and being constructed and arranged to move said wheel mountingportions generally downwardly relative to said track assembly so as toshift said distribution of vehicle weight from said track to saidwheels.
 70. A vehicle according to claim 69, wherein said axle assemblyhas a pair of arms extending away from each of said wheel mountingportions, respectively, said arms being hingedly connected to said frameto enable said wheel mounting portions to be moved generally verticallyrelative to said track assembly.
 71. A vehicle according to claim 70,wherein said power-operated structure is communicated to a supply ofpressurized fluid and is constructed and arranged to move said wheelsgenerally downwardly relative to said track assembly using saidpressurized fluid.
 72. A vehicle according to claim 71, wherein saidsupply of pressurized fluid is a supply of pressurized hydraulic liquidand wherein said power-operated structure includes hydraulic cylinders.73. A vehicle according to claim 72, wherein said power drive assemblyincludes a hydraulic pump constructed and arranged to pressurize saidsupply of pressurized liquid.
 74. A vehicle according to claim 73,wherein said axle assembly comprises a single axle extendingtransversely of said frame, said wheel portions being provided onopposing end portions of said axle and said axle, said arms beingconnected with said axle such that hinging movement of said arms movessaid axle and said wheels together generally vertically relative to saidtrack assembly.